The Microstrip Interdigital Filter (MIDF) which utilizes non-TEM propagation of microwave energy as disclosed in co-pending U.S. Pat. application Ser. No. 934,460 has a frequency characteristic generally indicated in FIG. 1c. While the filter performs well at its upper frequency skirt, the rejection of undesired energy below the intended passband has been found to be insufficient in some applications since the frequency drop-off at the lower frequency skirt is more gradual than the drop-off at the upper skirt.